Picking of stainless steels



Patented June 28, 1949 PICKLING OF STAINLESS STEELS John J. Healy, Jr.,Brookline, Maurice H. Taylor, Stoneham, and Furio A. Abbiati,Longmeadow, Mass., assignors to Monsanto Chemical Company, St. Louis,Mo., a corporation of Delaware No Drawing. Original application June 15,1940, Serial No. 340,772, now Patent No. 2,408,424, dated October 1,1946. Divided and this application August 14, 1946, Serial No. 690,600

2 Claims.

This invention relates to pickling solutions for steels and hasparticular application to the pickling of the so-called stainless steelswhich are in most cases essentially chromium-nickel-iron a1- loys. Thesealloys are characterized by their resistance to the action of commonpickling reagents used with success in connection with ordinary steels.

The main objects of the invention are to provide a pickling bath for theremova1 of heat scale from stainless steels which will not pit, etch orotherwise damage the surface of the alloy; which will impart to it thepleasing light gray, dull, satiny finish desired by manufacturers of ahigh grade product and resembling, to a remarkable degree, theappearance of a piece of dull-finished aluminum; which is relativelyeconomical to use; for which it is not difficult to obtain a suitablyresistant container; and from which there is no possibility of theevolution of nitrogen oxide ases.

It is common practice in the removal of scale from ordinary irons andsteels to dip them in dilute solutions of sulfuric or muriatic acid,either inhibited or uninhibited. Such treatment is usually satisfactory,especially with inhibited acids which have very little action on thesteel itself and which therefore result in a greatly decreasedconsumption of acid and much less damage to v the surface of the metal.In the case of stainless steels, however, this treatment is veryunsatisfactory since either the scale is not all removed if a shorttreatment is given, or in the case of a prolonged pickling the surfaceof the metal is badly pitted or etched, dark colored and veryunattractive in appearance. Even an inhibited acid, which will not pitthe stainless steel, leaves it with a dirty black surface.

The present invention is based on the discovery that mixtures of asoluble ferric salt, such as ferric sulfate, and hydrofluoric acid, orof ferric fluoride and a mineral acid, such as sulfuric acid, in aqueoussolution are very effective in removing scale from stainless steel andimparting to it the desired light gray, dull, aluminum-like finish.Other soluble ferric salts may be employed in combination withhydrofluoric acid, or with hydrogen fluoride salts or with fluoridesalts and mineral acids, so that broadly stated, this inventioncontemplates the use of mixtures of a soluble ferric salt and anionizable fluoride in baths having an acid reaction. Such solutions cansafely be used in lead-lined tanks, which would be attacked by nitricacid, and since they contain none of that acid, they cannot evolvenitrogen oxide gases,

small amounts of which are very deleterious to the workmen engaged inthe pickling operation.

Baths containing or developing both ferric and fluoride ions have beenused in the past for pickling stainless steels, but in these baths theferric salts present have not been the active pickling agent. Forexample, the Kiefer Patent No. 1,974,- 570 describes the pickling ofstainless steels with baths containing from 5 to 25% by volume ofcommercial nitric acid and up to 5% by volume of commercial hydrofluoricacid, while the Blaut and Lang Patent No. 2,115,005 describes theelectrolytic pickling of stainless steels with baths containing from 30to 80% of commercial concentrated sulfuric acid and from 2 to 20% ofcommercial concentrated hydrofluoric acid. Although the baths of both ofthese patents tend to develop ferric salts as the result of the picklingaction of the acids on the steel in the presence of an oxidizing agent(nitric acid or electricity), the resulting ferric salts are incapableof exerting any pickling action due to the continued presence of saidoxidizing agent. Thus, in accordance with the prior art baths, theamount of ferric ions continually increases during the life of the bath,While in the baths described herein a predetermined amount of ferricsalts is initially employed and this amount gradually decreases as thebath is used for its intended purpose, thereby showing that the ferricsalts employed in accordance with the present invention constitute anactive pickling agent.

Examples of combinations, with which satisfactory pickling of stainlesssteel may be accomplished according to this invention, follow: ferricsulfate and hydrofluoric acid; ferric fluoride and sulfuric acid; ferricfluoride and hydrofluoric acid; ferric sulfate and sodium fluoride;ferric sulfate, sodium fluoride and sulfuric acid; ferric sulfate andpotassium fluoride; ferric sulfate, potassium fluoride and sulfuricacid; ferric sulfate, calcium fluoride and sulfuric acid; ferric sulfateand sodium hydrogen fluoride; ferric sulfate and potassium hydrogenfluoride. In some cases when the scale formation is very light or of aneasily removable nature, solutions of ferric sulfate alone or ferricfluoride alone may be used. For example, such a scale is removed byimmersing the stainless steel in a solution containing 5% ferric sulfateat 80 C. for 2 /2 hours or at C. for 5 hours or in a solution containing5% ferric fluoride at C. for one hour or 30% ferric fluoride at 80 C.for 40 to minutes, or 15% ferric fluoride at 80 C. for 20 minutes.However, in order to obtain the greatest efficiency from the Per centFerric sulfate 1 to 30 Hydrofiuoric acid i .3 to

Water 98.7 to 60 It is preferable that the hydrofluoric acid be presentin a smaller amount than the ferric sulfate. Two specific formulaewithin the range above contain: 6% ferric sulfate with 1.5% hydrofluoricacid; and ferric sulfate with 5.7% hydrofluoric acid.

With these pickling baths temperatures between 25 and 95 C. may be useddepending on the condition of the scale and the article to be pickled.The duration of pickling may vary from 1 minute to 35 minutes, dependingon the temperature and concentration of the bath and the condition ofthe scale on the steel. Increasing the temperature decreases the time.

The following table will give some idea of the results obtained at 60 C.with solutions containing different amounts of ferric sulfate andhydrofluoric acid on an annealed stainless steel of the usual 18-8composition, i. e., containing 18% chromium and 8% nickel. Thepercentage figures under Composition of the bath are by weight. Forexample, 100 grams of the mixture designated 15% ferric sulfate and 5.7%hydrofluoric acid contains 15 grams of 100% ferric sulfate and 5.7 gramsof 100% hydrofluoric acid, or in other terms 16.3 grams of 92% ferricsulfate and 11.9 grams of 48% hydrofluoric acid.

The necessary time of immersion of the alloy in the ferricsulfate-hydrofluoric acid solutions may be substantially reducedby meansof a short preliminary treatment in a conventional acid pickling bath,not long enough to pit the metal but enough to partly loosen the scale.For example, in the case of a hot rolled unannealed 188 stainless steelwith a very difiicultly removable scale, a 5-minute dip in. an 8%sulfuric acid solution at 75 C. reduces the time in a 15% forricsulfate-5.7% hydrofluoric acid mixture from 20 to 3 minutes. Similarly,the time is cut from 20 minutes to 2 /2 minutes by a preliminary pickleof 30 minutes in an inhibited 8% sulfuric acid bath at 75 C. which doesnot pit, etch or otherwise damage the metal. The advantage of such aprocedure lies in the increased life of the relatively expensive ferricsulfate-hydrofluoric acid bath at the expense of a comparatively cheapsulfuric acid solution and in the marked reduction in time of picklingfrequently attained.

We have also found that the present invention may be used advantageouslyin the known process of pickling stainless steels electrolytically. Thesteel is suspended from the anode in a bath containing a suitableelectrolyte, such as dilute mineral acid, and the electrolytic picklingcarried out in the usual manner, after which it is possible to give it abright silvery, metallic appearance by immersion inany of the bathsforming the subject of the instant invention (e. g., a 5% ferricfluoride solution or a 15% ferric sulfate-5.7% hydrofluoric acidmixture).

In preparing a solution containing ferric sulfate and hydrofluoric acid,the following procedure has been found to be satisfactory. The ferricsulfate is added to the water in an amount suflicient to make thedesired strength of solution. Heat is applied and the temperature ismaintained at -90 C.-until with agitation the chemical is completelydissolved. Then the temperature is lowered to the constant point atwhich the steel is to be pickled. Finally the hydrofluoric acid is addedand the bath is ready for use. Commercial ferric sulfate containingabout F2(S'Os)3 may be used and the hydrofluoric acid may be the.commercial product containing 80% or more HF.

In the pickling operation, the stainless steel is kept immersed in theferric sulfate-hydrofluoric acid bath until the scale drops from thesteel or becomes sufficiently loosened to be easily and completelyremoved by a light brushing under Water or by a forceful stream ofwater.

The foregoing description has been directed more particularly to the useof solutions containing ferric sulfate and hydrofluoric acid in picklingstainless steel because this combination of chemicals seems to be themost satisfactory from a consideration of cost, results and otherfactors. However, the other previously mentioned combinations of ferricsalts with acids or with fluoride salts or with fluoride salts and acidmay be employed in a similar manner and. they constitute a part of thepresent invention.

The following are examples of successful application of these othercombinations at 60 C. on an annealed stainless steel of the 18-8composition, no preliminary pickle in sulfuric acid having been given.All percentage figures are on the basis of chemicals.

Time of Imniersion a l Composition of the Bath satisfactory ScaleRemoval 15% Ferric Sulfate, 5% Sulfuric Acid 1 hour. 5% Ferric Fluoride,2% Sulfuric Acid" 10 minutes. 4% Ferric Fluoride, 5% Sulfuric Acid 0.15% Ferric Fluoride, 5% Sulfuric Acid- 5 minutes. 8.5% Ferrlc Fluoride,11% Sulfuric Acid Do. 15% Ferric Fluoride, 15% Sulfuric Acid D0. 5%Ferric Fluoride, 1% Hydrofluoric Acid 35 minutes. 5% Ferric Fluoride,2.3% Hydrofluoric Acid. 20 minutes. 8.5% Ferric Fluoride, 5.7%Hydrofluoric Acid Do. 5% Ferric Sulfate, 4.2% Sodium Fluoride, 4.9% Sul-10 minutes.

furic Acid. 15% Ferric Sulfate, 10.5% Sodium Fluoride, 12.3% 5 minutes.

Sulfuric Acid. 5% Ferric Sulfate, 3.9% Calcium Fluoride, 4.9% Sul- 10minutes.

furic Acid. 15% Ferric Sulfate, 9.8% Calcium Fluoride, 12.3% 5 minutes.

Sulfuric Acid. 15% Ferric Sulfate, 5% Sodium Hydrogen F1uoride Do.

It should be kept in mind that the above combinations and percentagesare merely exemplary, and that it is possible to vary the percentage ofeach substance or mixture of substances employed without detractingappreciably from the benefits of this invention. For example, ferricfluoride and sulfuric acid may be used together in amounts varying from.5 to 20% ferric fluoride and from .7 to 25% sulfuric acid, while ferricfluoride alone is suitable in amounts varying from to 35%. Ferricfluoride and hydrofluoric acid may be used together in amounts varyingfrom .5 to 20% of ferric fluoride and .3 to of hydrofluoric acid; ferricsulfate, sodium fluoride, and sulfuric acid may be used in amountsvarying from 1 to 30% ferric sulfate, .6 to 25% sodium fluoride, and .7to 25 sulfuric acid, and if calcium fluoride is substituted for sodiumfluoride in the last mentioned mixture, the calcium fluoride may be usedin amounts varying from 1 to 30%. Where sodium hydrogen fluoride is usedas the acidifying agent together with ferric sulfate, the latter saltmay be present in amounts varying from 1 to 30%, While from .6 to 20% ofthe sodium hydrogen fluoride may be used.

It will be understood that this invention is applicable to stainlesssteels other than the 18% chromium-8% nickel all-0y of iron specificallyreferred to. For example, satisfactory results are obtained when it isapplied to steel alloys containing chromium, nickel and iron; chromiumand iron; and chromium, nickel, molybdenum and iron in varyingquantities, whether or not they also contain small, variable quantitiesof common stabilizing elements or impurities such as columbium,titanium, zirconium, selenium, tungsten, copper, silver, carbon,silicon, sulfur or phosphorus.

It is to be understood that although specific temperatures have beenstated herein and the time required for the treatment has beenindicated, these vary with the nature and condition of the article beingpickled, age of the bath and other factors. Although only the preferredforms of the invention have been presented in the foregoing description,it will be apparent to those skilled in the art that numerousmodifications may be made therein without departure from the spirit ofthe invention or the scope of the appended claims.

This application is a division of our co-pending application, Serial No.340,772, filed June 15, 1940, now Patent No. 2,408,424, issued October1, 1946, which is a continuation-in-part of application Serial No.140,988, filed May 5, 1937.

What we claim is:

1. An aqueous acid reacting non-electrolytic pickling bath for stainlesssteels consisting essentially of water and active pickling agentsconsisting of a non-oxidizing mineral acid and ferric fluoride, saidmineral acid being present in the bath in an amount varying between 0.7and 25% and said fluoride being present in an amount varying between 0.5and said bath being substantially free of nitric acid.

2. The method of pickling stainless steels which comprises immersingthem in an aqueous acid reacting non-electrolytic pickling bathconsisting essentially of water and active pickling agents consisting ofa non-oxidizing mineral acid and ferric fluoride, said mineral acidbeing present in the bath in an amount varying between 0.7 and and saidfluoride being present in an amount varying between 0.5 and 20%, saidbath being substantially free'of nitric acid.

JOHN J. HEALY, JR. MAURICE H. TAYLOR. FURIO A. ABBIATI.

REFERENCES CITED The following referenlces are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,824,932 Schermer Sept. 29, 19312,115,005 Blaut et al. Apr. 26, 1938 FOREIGN PATENTS Number Country Date8,236 Great Britain 1896

